Concrete walls, and other concrete structures and objects, traditionally are made by building a form or a mold. The forms and molds are usually made from wood, plywood, metal and other structural members. Unhardened (plastic) concrete is poured into the space defined by opposed spaced form members. Once the concrete hardens sufficiently, although not completely, the forms are removed leaving a concrete wall, or other concrete structure, structural member or concrete object, exposed to ambient temperatures. The unprotected concrete walls, structures or objects are then exposed to the elements during the remainder of the curing process. The exposure of the concrete to the elements, especially temperature variations, often makes the curing of the concrete a slow process and the ultimate strength difficult to control or predict. To compensate fir these losses, larger amounts of portland cement sometimes are used than otherwise would be necessary in order to insure sufficient concrete strength is achieved.
The curing of plastic concrete requires two elements, water and heat, to fully hydrate the cementitious material. The curing of plastic concrete is an exothermic process. This heat is produced by the hydration of the portland cement, or other cementitious materials, that make up the concrete. Initially, the hydration process produces a relatively large amount of heat. As the hydration process proceeds, the rate of hydration slows thereby reducing the rate of heat production. At the same time, moisture in the concrete is lost to the environment. If one monitors the temperature of concrete during the curing process, it produces a relatively large increase in temperature which then decreases rapidly over time. This chemical reaction is temperature dependent. That is, the hydration process, and consequently the strength gain, proceeds faster at higher temperature and slower at lower temperature. In traditional curing of concrete, first, the heat is lost which slows the hydration process; then, the moisture is lost making it difficult for the cementitious material to fully hydrate, and, therefore, impossible for the concrete to achieve its maxim strength.
Concrete in conventional concrete forms or molds is typically exposed to the elements. Conventional forms or molds provide little insulation to the concrete contained therein. Therefore, heat produced within the concrete form or mold due to the hydration process usually is lost through a conventional concrete form or mold relatively quickly. Thus, the temperature of the plastic concrete may initially rise 20 to 40° C., or more, above ambient temperature due to the initial hydration process and then fall relatively quickly to ambient temperature, such as within 12 to 36 hours. This initial relatively large temperature drop may result is concrete shrinkage and/or concrete cracking. The remainder of the curing process then proceeds at approximately ambient temperatures, because the relatively small amount of additional heat produced by the remaining hydration process is relatively quickly lost through the uninsulated concrete form or mold. The concrete is therefore subjected to the hourly or daily fluctuations of ambient temperature from hour-to-hour, from day-to-night and from day-to-day. Failure to cure the concrete under ideal temperature and moisture conditions affects the ultimate strength and durability of the concrete. In colder weather, concrete work may even come to a halt since concrete will freeze, or not gain much strength at all, at relatively low temperatures. By definition (ACI 306), cold weather conditions exist when “ . . . for more than 3 consecutive days, the average daily temperature is less than 40 degrees Fahrenheit and the air temperature is not greater than 50 degrees Fahrenheit for more than one-half of any 24 hour period.” Therefore, in order fbr hydration to take place, the temperature of concrete must be above 40° F.; below 40° F., the hydration process slows and at some point may stop altogether. It is typically recommended that concrete be moisture cured for 28 days to fully hydrate the concrete. However, this is seldom possible to achieve in commercial practice.
It is typical that concrete cylinders are poured from the same concrete mix used to form a watt, slab or other structure. These cylinders are then cured under water at 72 F. according to ASTM C-39. This method provides a standard by which the compressive strength of concrete can be determined. However, it bears little relationship to the concrete that is cured under ambient conditions.
Engius, Inc. has developed the IntelliCure Match concrete curing box. This concrete curing box comprises an insulated container with both heating and cooling elements disposed below the water level in the curing box. A temperature sensor disposed below the water level sends signals to a microprocessor. The microprocessor controls the amount of heating or cooling provided to the water in the curing box. A temperature sensor, such as the Intellirock sensor, is embedded in a curing concrete wall, slab or other concrete structure of interest that is subjected to the environment. The Intellirock sensor senses the actual temperature of the curing concrete. A signal is provided by the Intellirock sensor to the microprocessor. The microprocessor is programmed so that it controls the heating or cooling of the water in the curing box so that the temperature of the water matches the temperature of the curing concrete watt, slab or other concrete structure in which the Intellirock sensor is embedded. The IntelliCure Match concrete curing box therefore duplicates the temperature conditions actually experienced by the curing concrete wall, slab or other concrete structure of interest. The IntelliCure Match concrete curing box can also maintain the temperature of the water in the curing box at any desired constant temperature level.
Although the IntelliCure Match concrete curing box provides a useful function, it cannot control the temperature within the concrete curing box according to a predetermined temperature profile as a function of time. Curing concrete according to a predetermined temperature profile as a function of time provides desirable advantages, as disclosed in U.S. Pat. No. 8,545,749 (the disclosure of which is incorporated herein by reference in its entirety).
Therefore, it would be desirable to provide a concrete curing box that can cure concrete cylinders according to a predetermined temperature profile as a function of time. It would also be desirable to provide a concrete curing system that adjusts the temperature of curing concrete cylinders so that the temperature follows a predetermined temperature profile as a function of time.